Processes for the co-production of acetic acid and dimethyl ether may be carried out by the catalytic dehydration and hydrolysis of mixtures of methanol and methyl acetate. Such co-production processes are known from, for example WO 2011/027105, WO 2013/124404 and WO 2013/124423.
WO 2011/027105 describes a process for the production of acetic acid and dimethyl ether by contacting methanol and methyl acetate with a catalyst composition at a temperature in the range 140 to 250° C. wherein the catalyst composition contains a zeolite having a 2-dimensional channel system comprising at least one channel which has a 10-membered ring.
WO 2013/124404 describes a process for the co-production of acetic acid and dimethyl ether from a mixture of methanol and methyl acetate by contacting the mixture at a temperature from 200 to 260° C. with a catalyst composition comprising a zeolite possessing a 2-dimensional channel system comprising at least one channel having a 10-membered ring and a silica:alumina molar ratio of at least 22.
WO 2013/124423 describes a process for the production of acetic acid and dimethyl ether by contacting a mixture of methanol and methyl acetate with a zeolite catalyst wherein the zeolite has a 2-dimensional channel system comprising at least one channel having a 10-membered ring and having at least 5% of its cation exchange capacity occupied by one or more alkali metal cations.
In such dehydration-hydrolysis processes methanol is dehydrated to dimethyl ether and methyl acetate is hydrolysed to acetic acid. The reactions can be represented by:2 methanoldimethyl ether+watermethyl acetate+wateracetic acid+methanolThese reactions are equilibrium limited. The hydrolysis reaction consumes water and produces methanol and the dehydration reaction consumes methanol and produces water.
It has now been found that in the presence of solid acid catalysts, such as zeolites, the dehydration reaction is relatively slow and since water is consumed more quickly by the hydrolysis reaction, it is typically necessary to provide water to the system to maintain a steady-state concentration of water in the reaction. Water may be added to the dehydration-hydrolysis process as a component of process streams such as water-containing feed and recycle streams to the process.
In general, methanol obtained by commercial synthesis processes contains water and may also contain some dimethyl ether. The amount of water in the methanol product can vary depending upon such factors as the composition of the feed to the process and the process conditions, and in particular the amount of carbon dioxide employed in the methanol synthesis process.
Methyl acetate may be produced by processes for carbonylating ethers, such as the carbonylation of dimethyl ether with carbon monoxide, as described, for example in U.S. Pat. No. 7,465,822, WO 2008/132438 and WO 2008/132468. Although the principal reaction of dimethyl ether with carbon monoxide does not itself produce water, it has now been found that low levels of water can be produced via side-reactions taking place in the carbonylation process.
Thus the amount of water present in feeds, particularly in methanol feeds, to dehydration-hydrolysis processes may be sub-optimal for maintaining or optimising the operation of such processes. Furthermore, if such processes are operated as continuous processes, recycling of water-containing streams to the process can cause or contribute to fluctuations in the water concentration within the process. Water losses due to, for example leaks in the process can also create fluctuations in water concentration within the system. Such fluctuations need to be managed to maintain effective process operation.